A ladder that is 13 feet long is 5 feet from the base of a wall. how far up the wall does the ladder? reach?

This makes a triangle with one side of 13 one of 5 and the other x. In this case the x is 12 because 13,12,5 triangles are a special case, a Pythagorean triple. 5^2+12^2=13^2

25+144=169

169=169